T cells, a class of lymphocytes developed in the thymus, are responsible for cell- and antibody-mediated immunity. Two major classes of T cells perform very different functions. CD8+ cytotoxic T cells kill infected cells or eliminated microorganisms, and CD4+ helper T cells help activate responses of other white cells, mainly by secreting a variety of local mediators lymphokines, interleukins, or cytokines.
There are two subsets of helper T cells, i.e., Th1 and Th2 cells, which function differently. Th1 cells mediate cellular immunity. They are related to rheumatoid arthritis, non-obese diabetes, and colitis. Th2 cells mediate humoral immunity. They are related to humoral immune response, asthma, inflammation, and allergy. Both Th1 and Th2 cells are derived from naive helper T cells (Th0).